Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F

Research and Development of Zincoborates: Crystal Growth, Structural Chemistry and Physicochemical Properties

Borates have been regarded as a rich source of functional materials due to their diverse structures and wide applications. Therein, zincobrates have aroused intensive interest owing to the effective structural and functional regulation effects of the strong-bonded zinc cations. In recent decades, numerous zincoborates with special crystal structures were obtained, such as Cs₃Zn₆B₉O₂₁ and AZn₂BO₃X₂ (A = Na, K, Rb, NH₄; X = Cl, Br) series with KBe₂BO₃F₂-type layered structures were designed via substituting Be with Zn atoms, providing a feasible strategy to design promising non-linear optical materials; KZnB₃O₆ and Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄) with novel edge-sharing [BO₄]^5- tetrahedra were obtained under atmospheric pressure conditions, indicating that extreme conditions such as high pressure are not essential to obtain edge-sharing [BO₄]^5--containing borates; Ba₄K₂Zn₅(B₃O₆)₃(B₉O₁₉) and Ba₂KZn₃(B₃O₆)(B₆O₁₃) comprise two kinds of isolated polyborate anionic groups in one borate structure, which is rarely found in borates. Besides, many zincoborates emerged with particular physicochemical properties; for instance, Bi₂ZnOB₂O₆ and BaZnBO₃F are promising non-linear optical (NLO) materials; Zn₄B₆O₁₃ and KZnB₃O₆ possess anomalous thermal expansion properties, etc. In this review, the synthesis, crystal structure features and properties of representative zincoborates are summarized, which could provide significant guidance for the exploration and design of new zincoborates with special structures and excellent performance.

KNa4B2P3O13: A Deep-Ultraviolet Transparent Borophosphate Exhibiting Second-Harmonic Generation Response

A new deep-ultraviolet transparent alkali metal borophosphate KNa₄B₂P₃O₁₃ is synthesized by high-temperature flux method, and its properties were characterized by a combination of experimental and ab initio methods. This compound crystallizes in the noncentrosymmetric Pna2₁ space group and its framework is formed of one-dimensional infinite [B₂P₃O₁₃]_∞ chains consisting of apex-sharing [BO₄] and [PO₄] tetrahedra. KNa₄B₂P₃O₁₃ had a short ultraviolet edge (<190 nm) and exhibited a moderate powder second-harmonic generation signal (0.4 × KDP). The discovery of KNa₄B₂P₃O₁₃ enriches the structural diversity of borophosphates and would bring a more comprehensive understanding of the structure-properties relationship in deep-ultraviolet nonlinear optical crystals.

Rational structural design of benzothiazolium-based crystal HDB-T with high nonlinearity and efficient terahertz-wave generation

A new promising nonlinear optical crystal C25H25NO4S2 (HDB-T, P21 space group) exhibits significant macroscopic second-order optical nonlinearity about 1.5 times larger than that of the benchmark OH1 crystal benefiting from optimized orientated polar HDB cations. Widely tunable monochromatic terahertz-wave from 0.1 to 20 THz in the HDB-T crystal was realized for the first time.

Synthesis, crystal structures and nonlinear optical properties of β-RbCdI3·H2O and CsCdI3·H2O

Searching for new IR nonlinear optical (NLO) crystals with a high laser damage threshold (LDT) is still a challenge. In this paper, two new halides, β-RbCdI₃·H₂O and CsCdI₃·H₂O, have been synthesized by a conventional solution reaction. Both of them crystallize in the monoclinic space group Pc (no. 7). In their crystal structures, all the corner-sharing [CdI₄] tetrahedra connect with each other to form 1D chains, which further connect with [Rb(H₂O)]⁺/[Cs(H₂O)]⁺ complexes to form three-dimensional (3D) frameworks. All the polar [CdI₄]^2- groups align in such a way that results in a favorable net polarization. Powder second-harmonic generation (SHG) measurements indicate that β-RbCdI₃·H₂O and CsCdI₃·H₂O show phase-matchable SHG responses of 2.7 and 2.6 times that of KH₂PO₄ (KDP). The infrared transmission range can reach 21 μm. The UV-vis diffuse reflectance spectra indicate that the band gaps of β-RbCdI₃·H₂O and CsCdI₃·H₂O are about 3.26 and 3.54 eV, respectively. A preliminary measurement indicates that the LDT of their powders is about 90 and 100 MW cm^-2, respectively, suggesting that β-RbCdI₃·H₂O and CsCdI₃·H₂O are two new promising IR NLO materials. Their band structure and optical property calculations based on DFT methods are carried out.

Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical Materials: Expanding from Borates to Borate Fluorides to Fluorooxoborates

Coherent light radiation down to the deep-ultraviolet spectral range (λ < 200 nm) produced by common laser sources is extensively used in diverse fields ranging from ultrahigh-resolution photolithography to photochemical synthesis to high-precision microprocessing. Actually, it is hard to immediately obtain certain wavelengths, deep-ultraviolet coherent light in particular, from commercial laser sources. However, the direct second harmonic generation process governed in part by nonlinear optical crystals is a feasible and effective approach to generate deep-ultraviolet coherent light, which motivates chemists and materials scientists to find potential deep-ultraviolet nonlinear optical materials that can practically meet the scientific requirements. The research progress required to go from a new single-crystal structure to final device applications involves many pivotal steps and is highly time-consuming and challenging, and therefore, it is necessary to commence systematic studies aimed at shortening the research cycle and accelerating the rational design of deep-ultraviolet nonlinear optical materials. In this Account, we choose borates as raw materials because they have ever-greater possibilities to form desired noncentrosymmetric structures, wide optical transparency windows, rich structural chemistry, and also large polarizabilities to guarantee the coexistence of large second-order nonlinear optical coefficients and suitable birefringence. Besides, the effects of fluorine atoms on the structural chemistry and optical properties of borates have been summarized and analyzed. On the basis of these favorable influences, three specific rational design strategies, including experimental and theoretical methods, have been proposed in order to shorten the investigational cycle of discovering the new expected compounds with high physicochemical performances required for practical applications. In this way, the progress of searching for candidates for the next generation of deep-ultraviolet nonlinear optical materials was accelerated from borates to borate fluorides to fluorooxoborates with three effective strategies: (1) expansion of the frontier from borates to borate fluorides with the introduction of fluorine to achieve enhanced optical performance; (2) computer-assisted design of new deep-ultraviolet nonlinear optical materials with a newly introduced systematic global structure optimization method; and (3) expansion of the frontier from borate fluorides to fluorooxoborates by proposed functionalized oxyfluoride [BO _xF_{4- x}]^{( x+1)-} ( x = 1, 2, 3) chromophores to balance multiple criteria. The preliminary development of fluorooxoborates exhibiting high performance as a new fertile field to search for deep-ultraviolet nonlinear optical materials is highly encouraging and inspiring and can guide chemists and materials scientists with new directions and thoughts aimed at finding the next generation of practical deep-ultraviolet nonlinear optical materials.

A2SrMIVS4 (A = Li, Na; MIV = Ge, Sn) concurrently exhibiting wide bandgaps and good nonlinear optical responses as new potential infrared nonlinear optical materials

A series of A₂SrM^IVS₄ compounds concurrently exhibiting wide bandgaps and good NLO responses were proven as promising IR NLO materials.

Exploration of new nonlinear optical (NLO) materials is of importance for infrared (IR) applications. However, it is an extremely tough challenge to design and synthesize excellent IR NLO materials with optimal performances (e.g., concurrently a large NLO response and wide bandgap). Herein, four new mixed alkali/alkaline earth metal sulfides, A₂SrM^IVS₄ (A = Li, Na; M^IV = Ge, Sn), were successfully synthesized by a motif-optimization approach using the classical AgGaS₂ as a template. Note that all of them concurrently exhibit wide bandgaps (3.1–3.8 eV) and good NLO responses (0.5–0.8 × AgGaS₂) with phase-matching behavior, which satisfy the balance conditions (E_g ≥ 3.0 eV and d_ij ≥ 0.5 × benchmark AgGaS₂) of optical performances and hence are outstanding IR NLO materials. Remarkably, both of Na₂SrM^IVS₄ have the same structure without the structural transformation (Ge to Sn) in the reported related analogues and an interesting cation-dependent structural change is also found in Na₂M^IISnS₄ (M^II: Sr, R3c vs. Ba, I4[combining macron]2d). These results verify that the above design strategy of motif-optimization provides a feasible guide for the discovery of new IR NLO candidates and the A–AE–M–S (A = alkali metal; AE = alkaline-earth metal; M = Ga, In, Ge, Sn) system was identified as the preferred system for IR NLO materials.

Photo-tunable linear and nonlinear optical response of cyclophanediene-dihydropyrene photoswitches

Cyclophanediene (CPD)-dihydropyrene (DHP) is a negative T-type photochrome pair having a thermodynamically stable colored form, i.e., DHP. Interconversion between cyclophanediene and dihydropyrene is associated with significant changes in dipole moment, absorption wavelength and polarizability, which can impart substantial linear and nonlinear optical response. In this study, phototunable linear and nonlinear optical response of cyclophanediene-dihdyropyrene photoswitches is described. Cyclophanedienes and dihydropyrenes are functionalized at the internal position for maximum changes in volume and polarizability. The UV-Vis spectra are calculated at ɷB97XD, which was validated through a benchmark approach. An excellent correlation is observed between theoretical and experimental absorption spectra. Several CPD-DHP pairs have been recognized for clean interconversion in UV-Vis light without formation of a photostationary state. Nonlinear optical response of dihydropyrenes is remarkably higher than that of cyclophanedienes. In general, the calculated hyperpolarizability values of dihydropyrenes are about two to three orders of magnitude higher than those for cyclophanedienes. The trends in calculated hyperpolarizabilities are rationalized through two level method. The high nonlinear optical response of dihydropyrenes stems from low excitation energies. The remarkable difference in hyperpolarizabilities of these isomeric forms paves path for the design of phototunable nonlinear optical materials.

KLi(HC3N3O3)·2H2O: Solvent-drop Grinding Method toward the Hydro-isocyanurate Nonlinear Optical Crystal

A novel mixed alkali hydro-isocyanurate, KLi(HC₃N₃O₃)·2H₂O was first prepared in AOH-BOH-H₃C₃N₃O₃ (A/B = Li/Na/K/Rb/Cs) system via a solvent-drop grinding method. KLi(HC₃N₃O₃)·2H₂O shows a large second harmonic generation response (5.3 × KH₂PO₄) with an ultraviolet cutoff edge of 237 nm. More importantly, the bulk single crystal can be readily grown through water solution technique. Characterization of these crystals indicates that KLi(HC₃N₃O₃)·2H₂O has a high laser damage threshold (LDT) (4.76 GW/cm²) and exhibits a large birefringence (Δ n = 0.186@514 nm), which reduces Type I phase-matching to 246 nm.

Rational Design via Synergistic Combination Leads to an Outstanding Deep-Ultraviolet Birefringent Li2Na2B2O5 Material with an Unvalued B2O5 Functional Gene

Birefringent materials, the key components in modulating the polarization of light, are of great importance in optical communication and the laser industry. Limited by their transparency range, few birefringent materials can be practically used in the deep ultraviolet (DUV, λ < 200 nm) region. Different from the traditional BO₃- or B₃O₆-based DUV birefringent crystals, we propose a new functional gene, the B₂O₅ unit, for designing birefringent materials. Excitingly, the synergistic combination of Li₄B₂O₅ and Na₄B₂O₅ generates a new compound, Li₂Na₂B₂O₅, with enhanced optical properties. The Li₂Na₂B₂O₅ crystal with a size of up to 35 × 15 × 5 mm³ was grown by the top-seeded solution growth (TSSG) method, and its physicochemical properties were systematically characterized. Li₂Na₂B₂O₅ features a large amount of birefringence (0.095@532 nm), a short DUV cutoff edge (181 nm) with a high laser-induced damage threshold (LDT, 7.5 GW/cm² @1064 nm, 10 ns), favorable anisotropic thermal expansion (α_a/α_b = 5.6), and the lowest crystal growth temperature (<609 °C) among the commercial birefringent crystals. Moreover, the influences of the B₂O₅ structural configurations on the optical anisotropy were explored. The fascinating experimental results will provide a prominent DUV birefringent crystal and an effective synthesis strategy, which can facilitate the design of DUV birefringent materials.

Growth, Crystal Structures, and Characteristics of Li5ASrMB12O24 (A = Zn, Mg; M = Al, Ga) with [MB12O24] Frameworks

Two aluminoborates (Li₅ZnSrAlB₁₂O₂₄ and Li₅MgSrAlB₁₂O₂₄) and one galloborate (Li₅ZnSrGaB₁₂O₂₄) were successfully obtained for the first time, and the centimeter-sized crystals were grown by the Kyropoulos method. These crystals possess the [MB₁₂O₂₄] (M = Al, Ga) basic frameworks in their crystal structures, and the [GaB₁₂O₂₄] framework is a new framework in galloborates. The synthesis, single crystal growth, characterizations, and theoretical calculation are reported for these compounds.

Ba4Bi2(Si8-x B4+x O29) (x = 0.09): a new acentric metal borosilicate as a promising nonlinear optical material

A novel acentric metal borosilicate, Ba₄Bi₂(Si_8–xB_4+xO₂₉) (x = 0.09), exhibits the strongest NLO effects in borosilicates.

A new acentric metal borosilicate, namely Ba₄Bi₂(Si_8–xB_4+xO₂₉) (x = 0.09), has been synthesized by a standard solid-state reaction. The title compound crystallizes in noncentrosymmetric (NCS) space group I4[combining macron]2m with lattice parameters a = 11.0254(4) Å and c = 10.3961(9) Å. Structure refinements indicate that mixing of B atoms and Si atoms exists for a few atomic sites. In the “ideal” Ba₄Bi₂(Si₈B₄O₂₉), BO₄ or SiO₄ tetrahedra are inter-connected by corner-sharing to cyclic B₄O₁₂ or Si₄O₁₂ units. These B₄O₁₂ and Si₄O₁₂ units are further interconnected via corner-sharing to an “ideal” [Si₈B₄O₂₉]^14– 3D network. The Ba²⁺ and Bi³⁺ act as the counter cations and are located at the cavities of the structure. Ba₄Bi₂(Si_8–xB_4+xO₂₉) (x = 0.09) melts incongruently at a high temperature of 929 °C. Powder second-harmonic generation (SHG) measurements reveal that Ba₄Bi₂(Si_8–xB_4+xO₂₉) (x = 0.09) is a type I phase-matching compound with a good SHG response of about 5.1 times that of KDP (KH₂PO₄), which is the highest among the borosilicates reported so far. The SHG source has been studied by DFT theoretical calculations. Our preliminary results indicate that Ba₄Bi₂(Si_8–xB_4+xO₂₉) (x = 0.09) is a new second-order nonlinear-optical crystalline material candidate.

Structural insights into three phosphates with distinct polyanionic configurations

P–O units are shown to feature distinct polyanionic configurations from [PO₃]_∞ chains to [P₄O₁₂]⁴⁻ rings.

Ba4(BS3S)2S4: a new thioborate with unprecedented [BS3-S] and [S4] fundamental building blocks

The first BS-S group, BS₃S, with π-conjugation composed of planar BS₃ and a non-polar covalent S–S bond.

Pb10O4(BO3)3I3: a new noncentrosymmetric oxyborate iodide synthesized by the straightforward hydrothermal method

A new first reported oxyborate iodide Pb₁₀O₄(BO₃)₃I₃ has been synthesized by a straightforward hydrothermal method.

From centrosymmetric to noncentrosymmetric: cation-directed structural evolution in X3ZnB5O10 (X = Na, K, Rb) and Cs12Zn4(B5O10)4 crystals

A new Cs₁₂Zn₄(B₅O₁₀)₄ compound exhibits a short DUV cutoff edge (lower than 185 nm) and contains a 3D [ZnB₅O₁₀]_∞ network.

Sn2B7O12F with a 2∞[B14O24]6− layer constructed from the unprecedented [B7O16]11− fundamental building block

A new tin borate fluoride Sn₂B₇O₁₂F features a complicated 3D network composed of the unique [B₇O₁₆]¹¹⁻ FBBs and [Sn₄O₁₂F₂]¹⁸⁻ clusters.

Ba3B10O17F2·0.1KF: the first mixed alkali/alkaline-earth metal fluorooxoborate with unprecedented double-layered B–O/F anionic arrangement

The first mixed alkali/alkaline-earth metal fluorooxoborate Ba₃B₁₀O₁₇F₂·0.1KF with a unique double-layered B–O/F anionic framework was successfully synthesized and systemically characterized.